In previous studies, breathing 100% oxygen or carbogen (95% 02, 5% C02) increased the MR signal intensity in T2* weighted images of rodent tumors. Also, tumor oxygenation (p02) increased as increasing amounts of carbon dioxide (0%, 5%, 10%) were mixed with oxygen. We sought to determine if information about oxygenation and tumor necrosis can be extracted from MR images acquired during changes in breathing gas. Methods RIF-1 tumors were grown subcutaneously in the flanks of C3H mice (N=6). After the tumors reached a size of 200-400 mm3, they were imaged using the animal imaging system. A special respiration monitoring device was devised and used to ensure the animals toleratied the experiment. The mice were anesthetized with halothane, and the breathing mixture was toggled between air, 100% 02 (hyperoxia), carbogen (hypercapnia), and air mixed with 25% N2 to lower the oxygen concentration to 15% (hypoxia). Maps of the correlation between signal changes and gas changes were made. Registered p02 measurements were made using an Eppendorf polarographic oxygen electrode system. Discussion and Results In T2* weighted images, the signal in RIF- 1 tumors changed with hyperoxia while norinal and suspected necrotic regions did not respond. Hypoxic areas showed a strong response to hypercapnia. In two of the studies, the tumor did not respond to the hypoxic perturbation and mean p02 values of less than 2 mtorr were measured. Inhalation of oxygen, carbogen and 15% 02 can be used to perturb tumor physiology and provide valuable information. Qualitative comparison of the correlation maps to oxygen electrode measurements indicate that the MRI maps are sensitive to the physiological conditions within the tumor. Future experiments will increase the number of animals in the study and will.test the sensitivity of the MRI measurements to increases in tumor oxygenation induced by nicotinamide.